Typically, timing measurements can be performed by either using an internal device such as a delay cell, or using an external device such as a tester. Measurements using an internal device are usually performed by relative comparison. How long did an unknown delay time take relative to a potentially imprecise known delay. One of the problems occurs with the calibration of the “known” delay element to a precise value. Delay cells are generally limited in the accuracy of their measurements. The accuracy of a delay cell's performance can change with variations in the silicon manufacturing process, variations in operating voltage, and variations in operating temperature (PVT) of the delay cells. Likewise, the accuracy of the calibration of the delay cell can degrade the further the separation in time from when a delay is calibrated and when that same delay cell is used in making a time measurement.
Some prior art devices may become inaccurate unless complex methods to compensate for PVT are used. These methods may use searching algorithms for the calibration cycle, where a pulse is presented to the delay element and an external measuring device will “search” for the output transition edge. This requires significant time. Some internal calibration designs may choose to use digital delay lines (DLL's). However, DLL's usually occupy a significant amount of silicon area in an integrated circuit.
Measurements entirely through an external tester are subject to greater inaccuracies due to “edge placement uncertainty”. Further, the length of the cables may add an imprecise parameter into the measurement.
While the invention is subject to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. The invention should be understood to not be limited to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.